1. Field of the Invention
This invention relates generally to digitally-controlled microwave-frequency signal synthesizers and, more particularly, to a numerically-controlled fast-hopping frequency synthesizer employing a "Nyquist-Boundary Hopping" signal alias phase-locking scheme.
2. Description of the Related Art
Microwave voltage-controlled oscillators (VCOs) are well-known in the art and are seen in many microwave communications devices. For instance, in U.S. Pat. No. 3,593,204, Healey discloses a simple analog high-frequency VCO with a phase lock loop that does not employ any digital frequency synthesizer techniques. The analog VCO characteristics are slightly nonlinear and tend to drift with time and temperature and the thermal noise in any analog control voltage operates to frequency-modulate the VCO output frequency undesirably. Also, random frequency and phase modulation noise results from random noise in the oscillator and the limited Q of any analog frequency-control network.
These problems are commonly mitigated by phase-locking the VCO to a high-quality stable frequency reference. It is also often desired to cause the VCO to tune over a wide output frequency band. For instance, in a spread-spectrum communications transmitter, a microwave VCO may be required to rapidly tune over half an octave from about 2,500 MHz to 3,500 MHz to handle frequency-hops spanning most of the entire band without loss of phase coherency. This requires some arrangement to retain (or quickly regain) the microwave VCO phase lock to the lower (and more stable) reference frequency while moving from one microwave frequency to another in its band or while changing bands. As an illustration of the solutions proposed by various practitioners, U.S. Pat. No. 5,146,187 issued to Vandegraaf discloses an adaptive loop filter for synthesizer applications that improves phase-lock performance during band-switching.
Digital reference sources are sometimes preferred as reference sources because they are stable but they also are problematic because of the spurious noise created by the quantization effect during digital-to-analog conversion. Careful management of sampling and clocking frequencies can resolve this well-known problem. For instance, in U.S. Pat. No. 4,926,130, Weaver discloses a synchronous up-conversion technique that creates a high frequency by mixing a direct digital synthesizer (DDS) output with a stable high-frequency reference signal whose frequency is fixed at an integer (N) multiple of the frequency of a digital-to-analog converter (DAC) sampling clock. A second up-conversion stage may be added by mixing the intermediate result with another stable reference signal frequency fixed at an integral (M) multiple of the sampling clock frequency. This method provides a final stepped-up output signal frequency that retains the spectrum of the DDS reference without creating new spurs.
Weaver teaches away from using DDS alias frequencies to control higher output frequencies, citing unacceptable and unfilterable noise spurs in the output signal. But, in U.S. Pat. No. 5,886,752, Cross discloses a wideband phase and frequency modulator that employs one of the DDS alias frequency bands in a manner that permits him to broaden the available modulation bandwidth. Cross controls the quantization noise spurs by operating the DDS at a sampling clock frequency submultiple. He suggests that any tuning range desired can be had by adding another frequency synthesizer. Cross prefers using the first DDS alias band as the modulation reference to which the up-converted output frequency is locked using a mixer in the usual manner. However, like Weaver, Cross does not consider solutions to the problems seen in phase-locking the up-converted output signal to the DDS output reference.
A simple phase-locking technique is to sample the instantaneous VCO output amplitude at a submultiple of the VCO frequency and use the resulting phase error signal to control the VCO frequency. For instance, a sampling circuit commonly denominated a phase gate is often used for this purpose. It is an electronic switch that periodically closes for a very short period (e.g. 50 ps) at a rate precisely controlled by a reference clock signal. As is well-known, this switch will produce output signals (aliases) that correspond to the frequency differences between the microwave VCO output frequency and the several harmonics of the reference signal frequency. As is well-known, the phase gate sample time must be less than the reciprocal of the signal frequency for which a useful sample stream is desired.
For example, assume the signal from a microwave VCO designed to operate at 3,000 MHz is sent to a sampling gate that is driven by a stable 2 MHz reference signal. The difference signal from the phase gate includes a plurality of components at frequencies equal to the 3,000 MHz VCO output and an integral multiple of the 2 MHz reference. The 1,500th harmonic of the reference signal is at 3,000 MHz. If the VCO frequency is slightly high; say, 3,000.5 MHz, the phase gate output signal includes a corresponding 0.5 MHz component. The same component is obtained if the VCO frequency is 2,999.5 MHz. This 0.5 MHz signal can be passed through a suitable network to the VCO control terminal, causing the VCO frequency to eventually settle at exactly 3,000 MHz. The advantage of this well-known technique is that the excellent frequency drift and phase noise characteristics of a high-quality (low frequency) reference oscillator (such as a DDS) can be transferred with relatively simple hardware to a much less stable microwave VCO. The disadvantage is also well-known; that is the similar transfer of reference oscillator range and rate limitations.
Consider such a microwave VCO output frequency locked to the 30th harmonic of a 100 MHz reference oscillator. If the reference drifts by 1 kHz, the VCO output frequency drifts 30 kHz. Similarly, the VCO output phase noise is 30 times as bad as the reference oscillator phase noise impressed upon it over the bandwidth of the phase lock loop. This is why the reference signal must be very stable. But it is difficult and expensive to create a widely-tunable reference signal that is also very stable. Instead, the stable reference signal is tuned only over a small region so that the VCO may be locked on the Nth harmonic of the reference signal at one reference extreme and on the (N+1)th harmonic at the other. This permits tuning the microwave VCO by locking it to the reference frequency; so, for instance, a 3,000 MHz VCO locked to the 30.sup.th harmonic of a 100 MHz reference that is tunable over a .+-.3% range, can be tuned over the range from 2,910 to 3,090 MHz.
But this solution introduces another problem when it is desired to tune or sweep the microwave VCO beyond the range where the reference signal can tune. In one solution, the tuning range of the reference signal is extended a bit to allow some overlap of the lock range of the microwave VCO. Tuning over a wider range obliges the system to halt, unlock the microwave VCO, retune the reference oscillator to the other end of its range, and re-lock the microwave VCO to the next reference harmonic. Obtaining a lock by the microwave VCO to the correct harmonic of the reference can be a slow process that loses useful carrier cycles.
In U.S. Pat. No. 5,521,532, Gumm discloses a broadband DDS-controlled microwave signal source that locks the output frequency and phase to successive harmonic bands of a submultiple clock-frequency DDS output signal. This method solves the problem of how to extend the VCO frequency range while locking to a narrow-band reference oscillator but imposes some restrictions on certain other desirable capabilities. As the VCO frequency is swept monotonically, the DDS reference harmonic band is stepped without phase discontinuity from N to (N+1) or (N-1), retaining the VCO phase-lock as the DDS reference signal abruptly hops from one end of its frequency range to the other. Gumm discloses an elaborate digital frequency-hopping controller needed to force the DDS output frequency from one end to the other of its band without losing phase-continuity. However, while Gumm shows how to lock the broad-band VCO to the more stable narrow-band DDS reference during a leisurely and continuous sweep over many multiples of the DDS reference frequency band, he neither considers nor suggests how to maintain this phase lock during abrupt hops from one VCO frequency to another.
Because of these well-known difficulties, typical frequency-hopped microwave synthesizers are designed using mostly analog components. A stable reference frequency is used to derive each selected frequency output by frequency multiplication, fractional-N synthesis, or other approaches. For high-rate hopping, ultra high-speed PIN diodes or similar circuitry are used to quickly switch between multiples of this reference. A primary disadvantage of these approaches is that the analog circuitry is costly and yields are poor. Moreover, such solutions do not retain phase-continuity and each hop requires many output cycles to regain phase-lock. The DDS approaches discussed above are usually limited to hopping over a small portion of the output frequency band because the sample rates required to hop over larger distances exceed twice the Nyquist rate and require GaAs components or other expensive technologies.
It is desirable to resolve these problems by providing a microwave synthesizer that can be phase-continuously hopped over gigahertz ranges using only a mature inexpensive technology such as silicon CMOS or field-programable gate arrays (FPGAs). Until now, this has not been possible because of the well-known limitations discussed above. These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.